This application is based upon and claims the benefit of priority from Korean Patent Application No. 2003-38682, filed on Jun. 16, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The invention relates to a method and apparatus for correcting acceleration errors, and more particularly, to a method and apparatus for compensating for an error in position determined by an inertial navigation system (INS), an INS therefor, and a method which calculates a position in the INS.
2. Description of the Related Art
Typically, inertial sensors, including tri-axial acceleration sensors and tri-axial gyroscopes, are used to measure the position and orientation of a moving object of interest in a three-dimensional space. While INS orientation is obtained by solving differential equations to integrate angular velocities measured by the gyroscopes, position is obtained by removing gravity components from accelerations measured by the acceleration sensors, taking into account the INS orientation, and then calculating double integrals of the accelerations with respect to time. In this case, since INS orientation error derived from error in the gyroscope measurement increases with time, errors in the accelerations from which the gravity components have been removed also increase with time.
Thus, position errors of the INS due to errors in the acceleration sensor measurement and in the gyroscope measurement grow in proportion to the square of time and the cube of time, respectively. Since the INS errors dramatically increase over time in this way, it is very hard to calculate the position over a relatively long period of time using inertial sensors. The solution to overcome this problem is to correct errors in the acceleration sensors and gyroscopes, which is known in the art. Conventional methods for correcting errors in acceleration sensors will now be examined.
When the INS makes frequent stops during movement, zero velocity updates (ZUPTs or ZVUs), coordinate updates (CUPTs), and orientation updates are widely used to correct sensor errors. The ZUPTs are processes to reset a velocity of the INS to zero if the INS is detected to be stationary. The CUPTs are processes to reset an origin, when the INS reaches a predetermined position, to the predetermined position. The orientation update is a process to reset an orientation at the origin, when the INS takes a predetermined orientation, to the predetermined orientation. These methods can be performed in real time but only allow corrections at specific moments. Thus, errors begin to accumulate again after the corrections are performed. As shown in FIG. 1, when an INS stops again after ZUPTs are performed to correct velocity errors while the INS is held still, the velocity does not return to zero, which means that velocity errors increase over time until the velocity errors get re-corrected.
To overcome this problem, U.S. Pat. No. 6,292,751 discloses a method for correcting acceleration errors caused while an INS is in motion. This method assumes acceleration errors are constant. An acceleration error is calculated under a condition that a velocity is zero at the time when the INS starts to move and stops. Then, the error is subtracted from the measured acceleration and the resulting value is doubly integrated with respect to time in order to determine the position. FIG. 2 shows a process of compensating for velocity errors through acceleration error correction.
Referring to FIG. 2, the INS starts to move at time t1 and stops at time t2. This method assumes that an acceleration error during motion of INS is constant. FIG. 2A shows a measured acceleration A of the INS and an acceleration error d indicated by a dotted line. As shown in FIG. 2B, a velocity, which is obtained by integrating the acceleration A, is a first-order function. Since a velocity of the INS is 0 at time t1 before starting to move, the velocity of the INS at time t1 can be corrected to 0 as shown in FIG. 2C. Since the velocity is 0 at time t2, a velocity error within an interval between t1 and t2 indicated by the dotted line is subtracted from the velocity depicted by a solid line to produce the one shown in FIG. 2D. The resulting corrected velocity can be integrated to yield a position. However, since it is assumed that an acceleration error is constant, large discrepancies occur between the calculated position and the actual position.